In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is the antenna systems used by the radio access network (RAN) nodes in the communications network and/or the wireless devices (WDs) with which the RAN nodes communicate so as to provide network coverage for the WDs.
Advanced antenna systems may be used to significantly enhance performance of wireless communication systems in both uplink (UL, i.e., from WD to RAN node) and downlink (DL, i.e., from RAN node to WD). In the downlink, there are three basic approaches for utilizing the antenna: diversity, multiplexing and beam forming.
With beam forming, the radiation pattern of the antenna may be controlled by transmitting a signal from a plurality of elements with an element specific gain and phase. In this way, radiation patterns with different pointing directions and transmission and/or reception beam widths in both elevation and azimuth directions may be created.
With so called WD specific beam forming, (narrower) beams may be formed to specific WDs in order to increase the receive signal power in these specific WDs while at the same time controlling interference generated to other WDs receiving data transmission.
WD specific beam forming is not the only form of beam forming. In mobile broadband systems based e.g., on High Speed Packet Access (HSPA) and Long Term Evolution (LTE), a common reference signal is transmitted (e.g., on a Common Pilot Channel (CPICH) or as cell specific reference signals (CRS)). Such signals may be used by WDs both for measurements to select a RAN node to communicate with, as well as a demodulation reference signal for data to be received by both single and multiple WDs served by one RAN node. Often, the area where a specific cell specific reference signal is received with highest power (as compared to cell specific reference signals transmitted from other RAN nodes) is referred to as a cell, and beam forming of the cell specific reference signal may therefore be referred to as “cell shaping”.
One form of cell shaping used in existing cellular communications networks is electrical and mechanical down tilt, where coverage of the cell and interference between multiple cells can be adjusted by changing the elevation angle of the radiated beam (i.e., by changing the pointing direction of the antenna at the RAN node). Commonly used sector antennas employ a form of cell specific beam forming. More specifically, an array of vertically stacked antenna element connected with a passive feeder network may be used, and the feeder network may hence implement the beam forming at radio frequencies. In such cases, individual antenna elements, or groups of antenna elements are not visible at base band. However, in future advanced antenna systems, it is envisioned that elements or groups of elements will be controlled, and also observable, at baseband.
There exist a few automatic mechanisms that change the tilt setting based on some network measurements in order to adaptively change the antenna system to current network conditions. Existing mechanisms are designed for commonly used sector antennas of today where individual elements or groups of elements are not observable at baseband and do hence not exploit the full potential benefits of advanced antenna systems.
In addition to tilt, which is the pointing direction in elevation, the elevation beam width can also be adjusted. It may for example be increased for a smaller cell in a dense high rise area where the positions of WDs in the cell are dispersed within a wide range of elevation angles. Similarly, the horizontal pointing direction and beam width in azimuth may be changed to better match the distribution of traffic.
One potential issue with changing the radiation pattern is to ensure that coverage holes are not created. For example, it may be desirable to tilt down the antennas in several cells to reduce the interference between cells, but at the same time, there is a risk that coverage is lost, either in the sense that no cell specific signal is received at the some locations, with sufficient signal power, or that too many signals are received, so the signal to interference ratio becomes too low. This is a potential issue for reception of basic common signals that must be received by all WDs to be able to connect to the communications network, as well as reception of data with a certain minimum rate.
An existing solution is to try different settings, observe the communications network performance and keep the setting that gives the best performance. However, since WDs cannot be served in coverage holes, the poor performance for these WDs cannot be observed by the communications network. Current automatic tilt solutions try to mitigate this issue by changing the tilt in small steps, to avoid coverage holes. This however still does not a guarantee that coverage is maintained and it makes the adaptation slow.
Furthermore, there is also an inherent conflict when it comes to the period during which measurements are made. On one hand, it is desirable to measure for only short periods to enable decently rapid adaptation to perhaps even match fluctuations in the traffic, but on the other hand one would like to measure over somewhat longer time to capture performance that is relevant for all user position distributions.
Hence, there is still a need for an improved cell shaping in wireless communications networks.